Extensions allow declarative extensions to "JsRuntime" (ops, state, JS or middleware).
This allows for:
- `op_crates` to be plug-and-play & self-contained, reducing complexity leaked to consumers
- op middleware (like metrics_op) to be opt-in and for new middleware (unstable, tracing,...)
- `MainWorker` and `WebWorker` to be composable, allowing users to extend workers with their ops whilst benefiting from the other infrastructure (inspector, etc...)
In short extensions improve deno's modularity, reducing complexity and leaky abstractions for embedders and the internal codebase.
This commit starts splitting out the deno_web op crate into multiple
smaller crates. This commit splits out WebIDL and URL API, but in the
future I want to split out each spec into its own crate. That means we
will have (in rough order of loading): `webidl`, `dom`, `streams`,
`console`, `encoding`, `url`, `file`, `fetch`, `websocket`, and
`webgpu` crates.
